304 is superior to 409 SS. Any day of the week. I'm talking stainless types, not engines.
This is not to say 409 is bad. It's better than steel or aluminized steel in the long run. If you don't mind the price difference and can get it in 304, do it.
Stainless is not all created equal. 409 is what you will get most of the time with stainless exhausts, unless otherwise specified. IT WILL RUST in the snowbelt and if driven on salty roads, although not as fast as steel. 304 might discolor with heat, but that's about it. If you don't drive it in the weather or on salted roads, 409 should be ok. OTOH, 304 has about twice as much chromium and a butt load more nickel content. And pricier. Easiest way to tell the difference is stick a magnet to it. If it sticks, it's 409. If not, it's 304.